1. Field of the Invention
The present invention relates generally to a method and apparatus for detecting a splicing point on a magnetic medium on which control signals are recorded, and is more particularly suitable for use with a magnetic medium on which pulse-code-modulated (PCM) audio signals are recorded.
2. Description of the Prior Art
In one type of PCM recording system, sometimes known as the fixed head system, a PCM signal representing audio information is recorded in one or more information tracks extending along the longitudinal direction of a magnetic tape. Splicing may be performed on tapes recorded with such PCM signals in the same manner that it is performed upon magnetic tapes having standard analog audio signals recorded thereon, that is, by cutting two segments of tape in a direction generally perpendicular to their length and then by abutting the cut ends of the two segments, and fastening them together. To minimize error, it is known to record PCM audio signals in a series of data blocks, each of which includes a plurality of multi-bit information words, an error correcting code word for those information words, and a block synchronizing signal. Each of the information words in the data block may represent an analog signal level or the information word may be formed by multiplexing a specified bit from a plurality of different analog signal levels. The error correcting code within each data block is designed to correct for errors which may occur in the recording and reproducing of the pulse code modulated signals as a result of such factors as dust located upon the magnetic tape, signal noise produced by either the recording or reproducing electronics, or the like.
Unfortunately, the error correcting codes recorded with PCM signals might not provide optimum error correction for reproduction errors caused by or in the immediate vicinity of a splice on the magnetic tape. This is because the error correcting codes used with PCM signals usually have only a limited amount of informational redundancy and, thus may not have the capacity to correct for the amount of erroneous signal reproduction which often occurs in the vicinity of a splicing point. Furthermore, data blocks often are split at a splicing point so that a first incomplete portion of one data block will be connected at the splicing point to a second incomplete portion of another data block. As a result, it is often difficult, if not impossible, to reproduce correctly the audio signal recorded in the two split data blocks at the splicing point. This problem is compounded if the PCM signals within a data block are interleaved. Also, if the splicing of data blocks places data from one data block together with an unrelated error correcting code from another data block, there is a tendency of the reproducing apparatus to use that error correcting code to "correct" data to which it is not related.
As a result of the difficulties in preventing errors in the reproduction of a PCM signal in the vicinity of a splicing point, it has been proposed that a properly reproduced signal level occurring just before the splicing point be stored and used as a continuing signal level until a signal level can be properly reproduced after the splicing point. In accordance with another proposal, the signal level produced prior to the splicing point is faded out and the signal level produced after the splicing point is faded in, so as to provide a more continuous "blend" of reproduced signals, and thus less noticable signal loss at the splicing point. But, in order for such proposals to succeed, it is necessary that the location of the splicing point be detected accurately. Otherwise, a discontinuity in the signals is reproduced, which will result in the generation of an abnormal sound.
If the magnetic tape is sufficiently wide so as to contain a plurality of parallel longitudinal information tracks thereon, the splicing point can be detected by the presence of reproduction errors occurring in each of such tracks at the very same point. This detection is quite reliable, since a splice is one the few conditions that can cause errors to occur in a line extending all the way across a large number of parallel tracks. But when the width of the magnetic tape is relatively small, such that the tape contains a small number of parallel longitudinal information tracks, it is possible for conditions other than splices, such as scratches or dust upon the magnetic tape, to cause reproduction errors at the very same point in all of the tracks. Therefore, it is desirable to detect splicing points along a magnetic recording medium regardless of the width of that medium.